In order that a method of plasma treatment of materials should be utilized in metallurgy and could successfully compete with a conventional pyrometallurgical method of treatment of materials, a number of conditions must be fulfilled. Specifically, among these are the following:
1. Employment of plasma-forming mixtures composed of inexpensive and non-scarce gases capable of chemical reactions with treated materials, primarily those containing carbon and hydrogen and providing reducing reactions in melts.
2. Achievement of a long and reliable operating life of all the elements of respective electric arc devices, primarily the most thermally stressed ones, such as electrodes, at currents of not less than 500 amperes and at an power of a few hundreds of kilowatts.
Known in the prior art is a method of plasma treatment of materials, consisting in that introduced into the space between the electrodes of an electric arc device is a plasma-forming gas such as carbon dioxide (cf. British Pat. No. 874,970).
This method enables plasma treatment of materials to be conducted only in an oxidizing atmosphere since only carbon dioxide is used as a plasma-forming gas.
Besides, it is impossible to provide with this method a long and reliable operating life of electrodes at currents over 175 amperes and at a power above 10 kilowatts.
The operating life of electrodes can be extended by introducing carbon into a plasma-forming atmosphere.
Known in the art is a method of plasma treatment of materials (cf. British Pat. No. 1,101,279), consisting in introduction of carbon rods into the space between the electrodes of an electric arc device, with burning-off of which rods, a plasma-forming atmosphere being enriched with carbon which is further deposited on one or both electrodes.
In spite of the fact that this method makes it possible to extend the operating life of electrodes, it requires complex equipment as it needs additional power sources and mechanisms for feeding carbon rods into the space between the electrodes. PG,4
The operating life of the electrodes of an electric arc device can be extended in a simpler way, such as by introducing carbon-containing compounds into the plasma-forming atmosphere.
This is based on a well-known phenomenon of dissociation and pyrolysis of carbon-containing compounds at high temperatures of an electric arc with an accompanying evolution of free carbon.
Known in the art is a method of plasma treatment of materials (cf. RFG Pat. No. 1,206,531), consisting in that, introduced into the space between the electrodes of an electric arc device is a plasma-forming mixture composed of hydrogen with addition of hydrocarbons, the ratio between the number of atoms of carbon and that of hydrogen in the plasma-forming mixture being from 1:6 to 1:25, and the plasma-forming mixture is further supplied into an arc burning zone in the hollow cathode, the ratio between the operating voltage in volts and the operating current (in amperes) being from 5 to 15.
This method makes it possible to extend the operating life of the cathode from 30 seconds up to 100 hours at a current of 100 amperes and at an arc power of 100 kilowatts due to deposition of a carbon ring on the operating surface of the hollow cathode during arc burning, whereon a cathode drop region of an electric arc is directly located.
However, application of this method is limited since the composition of a plasma-forming mixture can include only hydrogen and hydrocarbons.
Besides, this method is unusable in a number of processes, such as welding, cutting, melting of electrically conductive materials, which require localization of electrode drop regions, one of them being located on a treated material.
Lastly, in this method, an arc current cannot exceed the value of the order of 100 amperes, while to provide stable striking and burning of an electric arc a power source with a voltage drop of not less than 1,000 volts is required, which certainly limits wide industrial application of this method.
Also known in the art is a method of plasma treatment of materials (cf. U.S. Pat. No. 3,246,115), consisting in that introduced into the space between the electrodes of an electric arc device is a plasma-forming mixture including a fuel gas (carbon-containing compounds) and an oxidizing agent.
This method can be employed only for gas-flame treatment of materials.
This is explained by the fact that the main energy release takes place, as with a conventional gas torch, on chemical burning-up of fuel carbon-containing gases in an oxidizing agent, these gases being supplied through the gas torch as a mixture. The electric arc burning between the tip of this gas torch and the nozzle of an electric arc device serves only to stabilize this burning and raise the temperature of combustion products.
Burning of carbon-containing compounds in an oxidizing agent, with a considerable excess of the latter, eliminates deposition of carbon on the electrodes of an electric arc device.
Known in the art is a method of plasma treatment of materials (cf. U.S. Pat. No. 3,307,011), consisting in that introduced into the space between the electrodes of an electric arc device is a plasma-forming mixture including carbon-containing compounds chosen from the class of hydrocarbons or carbon monoxide, which provide deposition of carbon at least on one of the electrodes of an electric arc device in the course of arc burning, and gases inert to the material of the electrodes.
Used as gases inert to particular materials are, for example, argon and helium when the electrode is made of carbon, copper and aluminium, or nitrogen and air when an electrode is made of copper.
This method makes it possible to reduce the erosion of electrodes and thus to extend their operating life with stable striking and burning of the arc at currents from 400 to 1,000 amperes.
However, this method does not provide operation of the electrodes under conditions of their constant restoration on retention of their parameters unchanged, i.e. does not provide equilibrium between the amount of carbon depositing on the operating surface and that of carbon carried away from this surface.
When realizing this method, the weight of the electrodes either constantly decreases (they are eroded), or constantly grows (they increase in size). In the first case the electrode is ultimately destroyed, and in the second one localization of a cathode drop region on the operating surface of the electrode is eliminated, which disturbs stable burning of the electric arc.
The aforementioned phenomenon can be explained by the following reasons firstly, the use of only the gases inert to the material of electrodes along with carbon-containing compounds in the composition of a plasma-forming mixture; secondly, the lack of quantitative relationship between the arc current and the flow rate of carbon-containing compounds depending on the composition of a plasma-forming mixture; and thirdly, the lack of relationship between the variation in the composition of a plasma-forming mixture and the time of arc burning.
Besides, this method does not provide high quality of plasma-arc welding, cutting and similar processes of treatment of electrically conductive materials due to the fact that, to realize this method, it is necessary to employ electric arc devices with the electrodes made either hollow (of copper) or lump (of carbon), which eliminates the possibility of localization of the arc column or plasma jet in the zone of treatment.